Raman gain is basically defined by comparing the optical intensity (or optical power) of a signal light when a Raman pumping light exists with the optical intensity (or optical power) of the signal light when a Raman pumping light does not exist. Such measuring methods are described for example in Japanese Laid-Open Patent Application Nos. 2002-296145 (paragraph 0005 and FIG. 2) and 2002-250674.
The Japanese Laid-Open Patent Application No. 2002-296145 further described a method to calculate Raman gain in an optical fiber transmission line in which power variation of a test light in the optical fiber transmission line is measured using OTDR (Optical Time Domain Reflectometry) for both state, one is to apply a Raman pumping light into the optical fiber transmission line and the other is not to apply any Raman pumping light into the optical fiber transmission line, and the Raman gain in the optical fiber transmission line is calculated from the difference of the propagation losses of both cases.
Japanese Laid-Open Patent Application No. 2002-202224 describes conditions for accurately measuring a Raman gain factor.
A conventional method to manually perform ON/OFF operation of a Raman pumping light is time-consuming and troublesome and accordingly incapable of efficiently measuring Raman gain. When an OTDR is used, the whole apparatus becomes very large.
In every conventional method, it is necessary to apply a high power pumping light of 100 mW or more into an optical fiber transmission line for detecting Raman gain with sufficient accuracy. When such a high power pumping light is input, an end surface of an optical fiber in an optical fiber transmission line is sometimes damaged. For instance, if there is dust or flaw on an end surface of an optical fiber, the surface might be destroyed by a high power pumping light.
Furthermore, it is very difficult for all conventional methods to measure Raman gain in a low Raman gain region.